J. Warren (Warren1983, pp. 41-43) generally dates Group 4 chalkoi with the legend ΣΙ after those with Σ, and the serifs also indicate a late date in the range ca. 330-200 BCE. She speculates that historical events at the end of the 4th c. or around the second quarter of the 3rd c. might have occasioned the shift.
“KC046 (Early-Middle Hellenistic bronze coin of Sikyon).” In Kenchreai Archaeological Archive, edited by J.L. Rife and S. Heath. The American Excavations at Kenchreai, 2013-2024. <http://kenchreai.org/gaek/kc046>